1. Field of the Invention
The present invention relates to a fuel cell, which is capable of preventing the dew condensation at a reaction gas header in a plate by humidifying the stacked cells in a fuel cell stack, wherein the heat discharged from the fuel cell stack is efficiently used for humidifying the cells. The present invention also relates to a method for operating such a fuel cell and to a fuel cell system, which are suitable for the operation of such a fuel cell.
2. Description of the Related Art
In a conventional polymer electrolyte fuel cell, an anode (fuel electrode) and cathode (air electrode) are deposited respectively on one surface and the other surface of a solid polymer electrolyte membrane to form a unified element as a cell (membrane electrode assembly), and a unit fuel cell is formed by clamping the cell between both a plate (a separator) having concave groove-shaped fuel gas channels on the surface facing the anode and another plate (another separator) having concave groove-shaped oxidant gas channels on the surface facing the cathode. Such a plurality of unit fuel cells are stacked and unified into a single unit by fastening the unit fuel cells together, using a bolt passing through the unit fuel cells and end plates adapted onto both ends thereof. Thus, a fuel cell stack is formed by the unit cells. In the operation, a fuel gas (hydrogen gas or reforming gas composed of mainly hydrogen) is supplied into the fuel gas channels and an oxidant gas (normally air) is supplied into the oxidant gas channels, so that a DC electric power is obtained from the electrochemical reaction which takes place via the solid polymer electrolyte membrane.
In such a polymer electrolyte fuel cell, it is required to humidify the solid polymer electrolyte membrane in order to obtain proper proton conductivity during a period for generating the electric power. In the prior art, therefore, the reaction gas (fuel gas and/or oxidant gas) is supplied into gas channels in the plate, after humidifying the reaction gas with a humidifier, so that the solid polymer electrolyte membrane is maintained in a moist state. In particular, it is preferable that the solid polymer electrolyte membrane is humidified with the reaction gas at the dew point equal to the temperature of the membrane or the cell temperature or higher in order to obtain sufficiently high proton conductivity.
Regarding the method for supplying a reaction gas having a dew point near the cell temperature, U.S. Pat. No. 5,382,478 discloses a method of humidifying the reaction gas, using a heat resulting from a fuel cell in the state where cooling water for the fuel cell comes into contact with the reaction gas via a water permeable membrane. Since, however, the evaporation heat significantly increases with the increase of the temperature, the fuel cell is mostly operated at a temperature of 65° C. to 70° C. A further increase of the temperature in the fuel cell requires a greater difference between the dew point of the reaction gas and the temperature of the fuel cell.
However, for example, when a reaction gas having a dew point near the cell temperature is supplied to in such a plate A as shown in FIG. 5, water vapor is condensed in a manifold B, and therefore the condensed water, i.e., the dew clogs the inlet of the gas channel C, thereby causing the flow of the reaction gas to be interrupted. Even when the dew point of the reaction gas is set at a temperature smaller than the cell temperature to some extent in order to avoid the above phenomenon, the dew condensation still takes place in gas channels C in response to the consumption of the reaction gas, so that the dew clogs the gas channels C and the supply of the reaction gas is suppressed. As a result, the reaction gas is not uniformly distributed, and therefore the amount of the reaction gas to be supplied to the electrode becomes insufficient and further the generation of the electric power is not normally carried out, thereby causing the performance of the fuel cell to be deteriorated. In particular, the gas flow resistance becomes larger in the vicinity of curved sections in the gas channels C, so that the condensed water is adhered thereto, thereby causing the gas channels to be clogged. In order to avoid this fact, Japanese Patent Publication No. 2761059 discloses a technical measure, in which, for example, S-shaped gas channels are replaced with those in the form of straight line and the condensed water is moved to downstream by supplying the reaction gas from top to bottom in the direction of gravity, and in which each water supplying channel is further interposed between the adjacent gas channels to enhance an efficiency in cooling the fuel cell stack. Moreover, as for means for preventing the deterioration of the power generation performance resulting form the condensed water, Japanese Unexamined Patent Application Publication No. 6-89730 discloses a technical measure, in which, for example, water absorbing elements are disposed in the gas channels and/or a dry gas is supplied at the middle portion in the gas channels to remove the condensed water.
In the above-described prior arts, the moisture content in the solid polymer electrolyte membrane and the temperature at the fuel cell stack can be maintained within predetermined ranges, so that an excellent responsibility of transferring to a heavy load for the fuel cell stack can be obtained and therefore a high output can be obtained in a short time, thereby enabling a stable operation to be ensured for such a load variation. However, regarding the suppression of dew condensation in the vicinity of the gas channel inlet in the case when the dew point of the reaction gas is increased up to a temperature near the cell temperature, satisfactory results cannot be always obtained. Regarding the countermeasure for the clogging of the reaction gas channels due to the water condensed in the gas channels, the above-described prior arts require either the insertion of water-absorbing material into the gas channels of the plate or the mounting of holes and channels for supplying dry gas in the middle portion of the gas channels. This causes to provide a complicate structure in the fuel cell and to require a lot of work for mounting these components and for manufacturing the fuel cell. It may be stated, therefore, that no sufficient countermeasure is yet introduced into the prior arts.
Regarding a fuel cell system in the prior art, it is necessary to dispose a heat source inside the fuel cell system in order to increase the dew point of the reaction gas near the cell temperature. Moreover, when the dew point of the reaction gas is higher than the temperature of the fuel cell stack, the dew condensation takes place without delay after the reaction gas is supplied to the fuel cell stack. Accordingly, it is necessary to control the relationship between the dew point of the reaction gas and the temperature of the fuel cell stack. The prior art method for humidifying the reaction gas, using the heat from the fuel cell, and bringing the cooling water for the fuel cell comes into contact with the reaction gas via a water permeable membrane, ensures that the dew point of the reaction gas is always smaller than the temperature of the fuel cell stack to some extent. In this case, however, the heat of evaporation strongly increases with the increase of the temperature, so that the fuel cell is operated at a temperature of 65° C. to 70° C. In addition, almost all the heat of the cooling water is used for humidification, so that it is difficult to recover the heat from the cooling water in the case of a cogeneration application.